Objective: To study the acute and chronic morphological effects of lesions which produce various degrees of axonal and synaptic degeneration and deafferentation of neurons in the mature and developing central nervous system. The areas studied will be the olfactory and trigeminal systems and both rats and cats will be used. Light and electron microscopy methods will be employed for control studies of normal synaptic relationships in these areas in mature and in newborn animals. Discrete lesions will be placed in various afferent and efferent pathways at similar ages and the animals will be allowed to survive for times appropriate to study the acute and long-term effects on synaptic organization and deafferentated neurons. Newborn animals receiving lesions will be allowed to survive to maturity and additional lesions of other afferent systems will be performed to analyze possible re- establishments of new synapses in the areas already partially deafferented. Emphasis will be directed toward the fine structural process of axonal and synaptic degeneration, synaptic modifications, effects on postsynaptic and perisynaptic regions and possible synaptic reorganization.